With the advent of bipolar plates, monopolar fuel cell stack assemblies considerably fell behind because of their complicated wiring in comparison with bipolar assemblies. This shall now be explained with reference to FIG. 1 on the basis of fundamental differences between a bipolar arrangement and a monopolar arrangement.
FIG. 1 shows schematic equivalent circuit diagrams of two equivalent series connections of individual voltage cells, the upper part of the figure representing a bipolar arrangement and the lower part of the figure a monopolar arrangement. In the bipolar arrangement the voltage cells are oriented such that a plus pole is opposite to a minus pole each time, which permits an easy wiring of the voltage cells. By contrast, in the monopolar arrangement the cells are oriented such that two plus poles or two minus poles are opposite in pairs each time. To connect the individual voltage cells in series, a comparatively troublesome wiring is needed, which partly explains why the bipolar arrangement has often been preferred considerably.
Irrespective of this, the monopolar arrangement certainly offers advantages over the bipolar arrangement. These shall now be explained hereinafter:
In bipolar stacks the cathode region of an individual cell is separated from the anode region of the directly neighboring individual cell by a separation plate (bipolar plate). In the stack said separation plates account for a large portion of the total stack volume which, based on the stack volume, deteriorates typical characteristics such as performance, etc. This drawback does not arise in a monopolar stack because two neighboring individual cells can here comprise a joint anode portion or cathode portion. Although the two anodes or cathodes of such a joint anode or cathode portion must be electrically isolated from one another, the fluid portions themselves need not be separated as long as the electrical conductivity of the cathode and anode fluids remains negligible, which (despite a certain conductivity, especially of the anode fluid) is normally the case.
The electrical wiring of monopolar stacks is carried out in the prior art either via direct contacting of the current discharging means by soldering (“direct adjoining soldering”) or by mounted contact terminals. The stack is clamped via the end plates by means of screws, tightening bands, or the like. As a consequence, current discharging means of different geometries or of a complicated wiring are required for a stack in the prior art. This is very disadvantageous in terms of assembly and proneness to assembling mistakes and is cost-intensive (e.g. several stamping tools are needed for different current discharging means). In addition, the former types are not suited for miniaturized fuel cell systems because they require a lot of constructional space. The usual clamping of the stack by means of screws requires a large minimum thickness of the end plates for realizing appropriately deep threaded bores. In other techniques, too, e.g. clamping by way of tightening bands, the end plates must absorb very great forces and therefore require a certain thickness for reasons of stability. On the whole, former clamping techniques are difficult and require a large volume and are thus hardly suited for miniaturized fuel cell systems.